This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-229210, filed on Jul. 28, 2000, the entire contents of which are incorporated by reference.
1. Field of the Invention
The present invention relates to a driving force control of a movable mold during a mold closing operation in an injection molding machine such as an electrically driven injection molding machine, particularly to an apparatus for protecting a mold in an injection molding machine including a toggle or another similar link mechanism for the mold closing operation and subsequent mold clamping operation.
2. Description of the Related Art
When a rigid foreign substance such as a small piece of a molding material is erroneously held between a movable mold and a fixed mold during a mold closing operation of an electrically driven injection molding machine, an irregular force is added to these molds. As a result, the molds will accidentally be broken, danger will be extended to an operator near the machine, and it is therefore important to control a driving force with respect to the movable mold. It is inherently necessary to directly set the driving force with respect to the movable mold, but the movable mold is not directly driven by a servomotor in the injection molding machine. A cross head connected to the servomotor via a link mechanism is driven for driving the movable mold to close the molds. Therefore, it is necessary to control the driving force of the movable mold as a control of the driving force of the cross head.
Particularly, when the movable mold is connected to the cross head via a complicated link mechanism such as a toggle mechanism, a very complicated calculation is required in estimating and controlling a thrust force to be supplied to the movable mold from the thrust force applied to the cross head, and a time consuming calculation processing is also required.
A conventional mold protection apparatus for use in closing the molds in the injection molding machine including the toggle mechanism is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-254218. In this disclosed prior art, a strain sensor for detecting a strain amount is disposed in any constituting member of the toggle mechanism and molds, the thrust force is estimated from the strain amount and limited in order to protect the molds by controlling the movement of the movable mold. However, since the strain sensor and the like are used in this system, a constitution becomes complicated and expensive.
Therefore, an object of the present invention is to provide an apparatus for protecting molds in an injection molding machine, in which the calculation for obtaining a movable mold thrust force from a cross head thrust force is simplified, the movable mold thrust force in a mold protection area during a mold closing operation is set to a value smaller than a mold protecting force when a cross head is moved forward, so that a driving force control is facilitated for a mold clamping operation, and a mold injection processing can be quickened.
According to one aspect of the present invention, there is provided an apparatus for protecting molds in an injection molding machine, comprising: a cross head; a driving apparatus for driving the cross head; a movable mold connected to the cross head via a link mechanism; and a controller for controlling a forward thrust force of the movable mold when the movable mold is brought into contact with a fixed mold to close the molds. The controller comprises a memory table in which a correspondence table showing a relation between a cross head position and a movable mold position is stored beforehand; and a driving unit which drives the cross head so that the movable mold moves with a thrust force preset to a mold protecting force or less in accordance with the position of the movable mold in a mold protection area using the memory table.
Moreover, a limit value of a minus thrust force on a backward side during deceleration of the cross head during mold closing until the movable mold contacts the fixed mold is set to a maximum driving force of the cross head.
With the constitution, the memory table with the correspondence table denoting a relation between the cross head position and movable mold position stored beforehand therein is used to limit the driving force applied to the movable mold in the mold protection area during a forward moving operation of the movable mold in accordance with a predetermined magnified force ratio transmitted to the movable mold from the cross head via the link mechanism. Thereby, the thrust force calculation is simplified, and the molds can easily be protected during a mold clamping operation while the movable mold is driven at a high speed.
Moreover, even when the movable mold is moved at a high speed, the speed can effectively be reduced with respect to a mold clamping position, an inertial force of the movable mold can sufficiently be controlled, and the mold protection operation can easily be performed at a high speed during the mold clamping operation.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.